Proceedings of the Sudden Oak Death Third Science Symposium
Linking Sudden Oak Death With Spatial
Economic Value Transfer1
Tom Holmes2 and Bill Smith2
Abstract
Sudden oak death (caused by Phytophthora ramorum) is currently having a dramatic impact
on the flow of ecosystem services provided by trees and forests in California. Timber species
in California are not thought to be at risk of mortality from this pathogen and, consequently,
economic impacts accrue to non-market values of trees such as aesthetics, shading, and the
knowledge that healthy forest ecosystems exist. Because non-market valuation studies are
expensive to design and implement, we propose that spatial benefit transfer methods can be
used as a pragmatic means for obtaining second-best estimates of the economic damages
associated with P. ramorum. Economic damages to residential property values and public
forest land are identified as a major concern.
Key words: Economic impacts, non-market values, value transfer, hedonic property value,
contingent valuation, existence value, GIS.
Introduction
Forests provide suitable habitat for an assortment of invading organisms (Liebhold
and others 1995) and invasive species have been ranked as one of the four critical
threats to our Nation’s forest ecosystems by the Chief of the United States
Department of Agriculture-Forest Service (USDA-FS) (USDA Forest Service 2004).
Although most people might argue that it is laudable to counter threats to the
structure and functioning of forest ecosystems, relatively few exotic organisms
become a major pest (Williamson 1996). It is the main thesis of this paper that
decisions regarding budget allocations and the targeting of forest protection efforts
would benefit from a clear understanding of the costs and benefits of invasive forest
pest management (Holmes and others 2007). Interventions designed to mitigate
damages from exotic forest pests are costly - the USDA-FS spent $95.1 million
dollars for the management of invasive forest pests in fiscal year 2005 (USDA Forest
Service 2005, p. 14 to 55). However, very little is known about the magnitude of
economic damages caused by exotic forest pests, or the efficacy of the money spent
on pest control. This lack of knowledge impedes economic analyses of pest
management programs and it remains unclear whether current expenditures on
invasive forest pests are too little, too large, or about right.
Economic damages are incurred when invasive forest pests alter the flow of
ecosystem services provided by trees and forests which are valued by people. Forest
ecosystem services can be broadly categorized as services that are traded in markets
(such as timber) and non-market-based services (such as aesthetically pleasing
views). Market and non-market values can be evaluated using consumer and
1
A version of this paper was presented at the Sudden Oak Death Third Science Symposium, March 5–9,
2007, Santa Rosa, California.
2
Forestry Sciences Lab, Southern Research Station, USDA Forest Service, PO Box 12254, Research
Triangle Park, NC 27709.
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GENERAL TECHNICAL REPORT PSW-GTR-214
producer surplus concepts drawn from welfare economic theory. In instances where
data are not available on the supply of, or demand for, economic goods and services,
other non-welfare-theoretic methods may be adequate. Although it is often tempting
to sum various economic measures to arrive at an aggregate estimate of damage, it is
important not to sum economic measures that are based on different conceptual
foundations. Likewise, it may not be meaningful to compare damage estimates
computed using widely divergent economic methods.
Perhaps the most widely cited estimate of the aggregate damages from exotic forest
pests ($4 billion per year) is due to Pimentel and others (2000). Their measure was
computed by multiplying the price of timber by an estimate of the annual quantity of
timber destroyed using pest loss rates estimated for Canadian forests. Their measure
does not include the broader impacts of forest pests on non-market economic values
and is therefore biased downwards, perhaps severely. For example, using such a
methodology, the forest ecosystem disturbances occurring in California and Oregon
from P. ramorum infections would not be causing any economic damage as
important timber species in this region are not thought to be at risk of loss from this
pathogen (Rizzo and others 2005). Other approaches are clearly required to estimate
the economic damages associated with the loss of hundreds of thousands of trees in
private and public landscapes currently being killed by this pathogen.
Spatial Benefit Transfer Methods
In this paper, we propose the use of benefit transfer methods for estimating the nonmarket value losses attributable to the spread of P. ramorum in California. The logic
for using benefit transfer is pragmatic - non-market valuation studies are expensive to
design and implement. When funds are unavailable for collecting primary data for a
non-market valuation study, benefit transfer may provide a reasonable second-best
alternative.
Broadly speaking, a benefit transfer uses estimates of economic value derived from
research conducted at a study site and transfers the estimates to a policy site of
interest. Various methods are available for conducting a benefit transfer study and
which method is used depends upon the data available and the goals for conducting
the transfer (Rosenberger and Loomis 2003). The value transfer approach directly
applies summary statistics from the original research to the policy site. This approach
would typically use a point estimate, some measure of central tendency, or an
administratively approved estimate of value. The function transfer approach is more
technically demanding than a simple value transfer as it involves the transfer of
statistical models defining functional relationships between variables. Examples of
this method would include the use of demand or willingness-to-pay (WTP) functions
as well as meta-analysis regression functions. Function transfers may be more
informative than value transfers as they can facilitate adjustments that account for
salient characteristics at the policy site. Both the value and function transfer
approaches require the acquisition of data from existing studies which may be
identified in published and unpublished literatures.
The damage caused by invasive species is intrinsically spatial in nature and
identifying the spatial incidence of damages improves the calibration of benefit
transfer estimates, especially when damages occur over broad geographic areas and
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Proceedings of the Sudden Oak Death Third Science Symposium
land uses. Although most benefit transfer studies have been non-spatial, the
increasing prominence of Geographic Information System (GIS) technology is
advancing the ability to conduct spatial value transfers. Eade and Moran (1996)
provide an early example of spatial value transfer by mapping the quality of natural
capital assets in Rio Branco, Belize and then re-calibrating benefit transfer estimates
using the quality maps. Bateman and others (2002) describe how travel cost
assessments of recreational value can be transferred using the analytic capabilities of
a GIS. More recently, Troy and Wilson (2006) developed a typology of land cover
and aquatic resources which were spatially linked with economic valuation studies in
three study sites.
At a landscape scale, trees and forests provide a heterogeneous array of ecosystem
services. Humans recognize and value some sub-set of this array. A linkage between
ecosystem services and economic value is provided by the recognition that human
land uses reflect decisions about the provision of goods and services valued by
people. For example, people choose to live in certain areas of the overall landscape,
work in other areas, and set aside special locations for public use or scientific study.
Within each of these land uses, forest ecosystem goods and services (such as trees)
are managed and valued differently. Thus, one method of linking the flow of forest
ecosystem services with economic values across a landscape is to identify the
relationship between ecosystem characteristics, land-uses, and economic value.
Recognizing that economic values are not absolute, but rather represent the value of
changes from a starting point or status quo state to an alternate state, the approach we
recommend in this paper is to evaluate how changes in forest ecosystem
characteristics alter the value of non-market good and service flows within separable
land uses. In particular, the spatial benefit transfer method we propose is based upon
a simple algorithm for identifying linkages between land uses, forest disturbances
caused by P. ramorum and non-market economic damages:
Economic damagei = [( Areai ) ∩ ( SOD)] * (value∆ i ) ∀ i
(1)
where Areai is the area in land use type i, SOD is the area of P. ramorum infestation
value∆i is the value change in land use type i resulting from P. ramorum infection,
and ∩ is the intersection operator. It is important to note in equation (1) that
economic impacts are computed separately for each land use category and yet are not
summed across land uses. This is because economic estimates of value changes in
forest health provided by the economics literatures are partial (rather than general)
equilibrium impacts. Because the degree of interaction between forest health value on
different classes of land use are unknown, summing the values across land use
classes would lead to a biased estimate of aggregate impact unless the degree of
interaction between land use classes is known to be zero. As we are unaware of any
studies estimating the linkages between forest health values across land uses, we
recommend evaluating each land-use class separately. None-the-less, categorical
estimates can provide an indication of the relative importance of changes in forest
health on various land use classes of interest as long as estimates are based on the
same theoretical foundation.
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GENERAL TECHNICAL REPORT PSW-GTR-214
Data Requirements
Three types of data are required to implement the economic value transfer method as
shown in the above equation: (1) economic value estimates drawn from studies
conducted at original study sites, (2) GIS data on forest health conditions in the
policy area, (3) GIS data on land uses in the policy area. Each data requirement will
be discussed in turn.
Economic Value Transfer Data
The economic literature on the non-market economic value of trees and forests has
been focused primarily on the value of trees in residential and public landscapes.
Consequently, the studies we briefly review are thought to be representative of the
non-market economic values that trees add to urban and public forests.
Data Assessed With the Hedonic Property Value Method
One method of evaluating the economic impacts of invasive species on residential
properties is to use econometric methods to tease out the change in property value
due to a change in tree numbers or a decline in tree health while controlling for all
other factors influencing housing prices. This method, known as the hedonic property
value method (HPM), has the advantage of utilizing data on actual economic
transactions. Several economic studies have been conducted using (HPM) to estimate
the contribution that trees make to private property values in residential landscapes
(table 1). An early study was provided by Morales and others (1976) who found that
tree cover added about 6 percent to property values in Manchester, Connecticut. A
similar result was reported by Anderson and Cordell (1988) who concluded that trees
in the front yard of homes in Athens, Georgia added about 3 to 5 percent to housing
prices relative to houses without trees (and each large front yard tree added about
0.88 percent to property value.)
Payne and others (1973) estimated that, on average, each large tree (> 6 in. [15.24
cm] dbh) added about $270 to property value in Amherst, Massachusetts. They also
reported that the value arc elasticity (ε) for trees was 0.24. This means that a 1
percent increase (decrease) in the number of trees increases (decreases) property
value by 0.24 percent. A somewhat larger arc elasticity (ε = 0.66) was found by
Holmes and others (2006) for healthy hemlock trees on properties in Sparta, New
Jersey.
In general, the available economic literature indicates that trees add, on average, from
1 to 5 percent to the value of private property in a residential setting. This stylized
fact is consistent with Dombrow and others (2000) who, using tree appraisal
methods, found that mature trees contributed about 2 percent to the value of singlefamily homes in Baton Rouge, Louisiana. Note that these values provide an estimate
of the value of trees as a “stock” or asset value which, in turn, provides a flow of
services over time. An estimate of the “flow” (for example, annual) value of
residential trees would need to adjust these estimates for the length of time over
which trees provide this service.
If trees make a positive contribution to property values then, presumably, factors that
cause the demise of trees will reduce property values. However, it is not obvious that
the loss in value due to either a loss in tree health or numbers will be symmetric with
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an equivalent gain in health or numbers. Consequently, measures of the loss in
property value due to a loss of tree health would improve the precision of economic
damage estimates. We are only aware of two studies that directly estimate the loss in
private property values from a decline in forest health. Holmes and others (2006)
studied the impact of an exotic forest insect, the hemlock woolly adelgid, on property
values in Sparta, New Jersey (table 1). They found that hemlocks with declining
health decreased property values both for the property owner as well as having spillover impacts on other properties in the neighborhood. Notably the absolute value of
the (negative) value arc elasticity (ε = 0.96) for hemlocks in declining health was
greater than the arc elasticity (positive) for healthy hemlocks (ε = 0.66). In a related
study, conducted with a richer dataset in West Milford, New Jersey, Huggett and
others (2007) found that property values decreased substantially during the later
stages of hemlock woolly adelgid infestations when most hemlocks were either dead
or dying. Their estimates show that for each 1 acre (0.4 ha) increase in dead and
dying hemlocks, property values were reduced by about 8 percent in this housing
market. Further, this study indicates that the major economic impact of a change in
forest health occurs when most of the host trees are dead or dying. Thus, the time
dimension is critical when estimating economic impacts.
Thompson and others (1999) used the hedonic pricing method to evaluate the change
in private property value in Lake Tahoe, California resulting from tree mortality due
to native insect infestations. In particular, their methodology estimated the change in
property value due to thinning and removal of dead and dying trees. They identified
substantial gains in value (1 to 30 percent) which were attributed to both improved
visibility and a reduction of fire hazard.
Data Assessed With the Contingent Valuation Method
A second method for estimating the economic value of changes in forest health is to
use surveys to elicit stated values from a random sample of the population. This
method, known as the contingent valuation method (CVM), has the advantage that it
is the only method capable of eliciting existence values, or the value of knowing a
resource (such as a healthy forest) exists, even if one never anticipates visiting that
resource. Although the CVM is typically used to estimate the non-market economic
value of public resources, Walsh and others (1981a, 1981b) used this method to
estimate the reduction in private property values in the Rocky Mountains of Colorado
due to infestations of native forest insects. Contingent valuation surveys were
implemented via interviews both with private property owners and with real estate
appraisers. Estimates of value arc elasticity were similar across groups and indicated
that a 1 percent increase in the area of visible damage from native forest pests results
in 2.3 to 2.5 percent reduction in property value.
The CVM has also been used to assess the economic impacts of changes in forest
health on public forest land. Several analyses have been conducted using contingent
valuation data that were collected to evaluate public WTP to protect high elevation
spruce-fir forests in the southern Appalachian Mountains from the exotic balsam
woolly adelgid (BWA). These analyses (Haefele and others1992, Holmes and
Kramer 1995, Holmes and Kramer 1996, Kramer and others 2003) indicated that: (1)
median annual household WTP for BWA protection programs ranges from $28 to
$36 and, (2) existence value is an important component of total forest value.
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GENERAL TECHNICAL REPORT PSW-GTR-214
Table 1—Sources for non-market economic valuation of trees and forests
Value type &
location
Private property
value
Healthy trees
Connecticut
Georgia
Massachusetts
New Jersey
Louisiana
Dead and dying trees
New Jersey
New Jersey
Colorado
Colorado
California
Public forest value
Healthy trees
California
California
Dead and dying trees
North Carolina
Annual
benefit/ loss
Economic
unit
Economic
method
6 percent house
value
3 to 5 percent
house value
(0.88 percent per
large front yard
tree)
ε = 0.24 house
value
ε = 0.66 house
value
2 percent house
value
Urban forest
tree numbers
Urban forest
tree numbers
HPM
Urban forest
tree numbers
Ex-urban forest
tree area
Urban forest
tree numbers
HPM
ε = 0.96 house
value
8 percent house
value (per ac.)
ε = 2.27 house
value
ε = 2.48 house
value
1 to 30 percent
house value
Ex-urban tree
area
Ex-urban tree
area
Rural visible
damage
Rural visible
damage
Thin unhealthy
trees in resort
area
HPM
$477/ tree
Urban forest
tree numbers
Urban forest
tree numbers
CTLA
National
Forest/ Park
area
CVM –
households
$27.12/ resident
$54.33/ tree
Median WTP =
$28 to $36/ hhd.
HPM
HPM
Tree appraisal
HPM
CVM – owners
CVM –
appraisers
HPM
Various
Data source
Morales and
others (1976)
Anderson &
Cordell (1988)
Payne and others
(1973)
Holmes and others
(2006)
Dombrow and
others (2000)
Holmes and others
(2006)
Huggett and
others (in press)
Walsh and others
(1981a)
Walsh and others
(1981b)
Thompson and
others (1999)
Nowak and others
(2002)
McPherson and
others (1999)
Holmes and
Kramer (1996);
Kramer and others
(2003)
Colorado
Mean WTP =
National Forest CVM Walsh and others
$52/ hhd.
area
households
(1990)
Note: HPM refers to the hedonic price method, CVM refers to the contingent valuation method, CTLA
refers to the Council of Tree and Landscape Appraisers.
In a similar study, Walsh and others (1990) used the contingent valuation method to
estimate public WTP to protect national forests in Colorado from native insect
infestations. Their value estimate (average annual household WTP = $52) was similar
to the values reported for the southern Appalachian Mountains. In addition, they
reported that non-use values dominated the estimates of use value. Taken together,
these studies indicate that the public holds a significant willingness to pay for forest
health protection on public lands, and that non-use values are an important
component of total economic value.
It should be pointed out that the contingent valuation method is not without its critics,
and that criticism stems largely from the fact that WTP estimates are based on stated
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Proceedings of the Sudden Oak Death Third Science Symposium
intentions rather than actual expenditures. Further, forest health protection studies
conducted using the contingent valuation method have not fully incorporated the
dynamic nature of forest disturbances. That is, forest succession will follow some
(perhaps largely unknown) pathway following mortality due to either native or exotic
forest pests. Thus, WTP estimates need to take this dynamic into consideration.
Finally, because WTP estimates are typically estimated for households, an estimate
of aggregate impact requires that the “extent of the market”, or the number of
households holding a positive WTP for the public forest at risk, be determined.
Data Assessed With Accounting Methods
Finally, it should be recognized that a few studies have been published that seek to
estimate the value of trees in the urban forest using accounting methods. These
studies are conducted using either a tree appraisal approach (for example, Nowak and
others 2002) or mixed methods (for example, McPherson and others 1999). Tree
appraisal methods use a formula to estimate tree value based on the physical
characteristics of a tree. Although tree appraisal methods are not based on theoretic
economic models, they appear to provide fairly similar estimates to the hedonic
pricing method when applied to residential properties. The application of tree
appraisal methods to other land uses, such as the value of street trees, cannot be
similarly compared to theoretic economic models because we are unaware of such
studies. Other studies that use mixed methods for valuation estimate an aggregate
value of trees in the urban forest by summing values for the various services that
trees provide, such as aesthetics, shading, and moderating water runoff. As noted
above, the summation of values estimated using dissimilar methods (such as hedonic
property values and estimates of energy savings from the cooling function of trees in
summer) is not consistent with standard economic theory.
Forest Health and Land Use GIS Data
A review of the non-market economic value studies above provides us with an
indication of the types of forest cover and land use data that would be required to
conduct an economic value transfer study. To match up with the economic value
data, GIS forest health data would need to be available for the policy site(s) that
include the spatial location and area, number, or percentage of trees that are hosts for
the pathogen, metrics concerning the area, number, or percentage of trees that are
currently infected, and the anticipated rate of spread. In addition, as the economic
literature suggests that economic impacts are most pronounced during the period of
time when host trees are dead or dying, spatially reference data would be required for
the timing and location of tree mortality induced by P. ramorum dispersal.
The land use data required for economic value transfer would include GIS data
depicting the location and area of tree hosts on land uses such as urban residential
properties (for transfer of HPM values) and public lands such as local, state, and
national parks and forests with tree species that are at risk of mortality from
P. ramorum (for transfer of CVM values). Remote sensing data have proven to be
successful in mapping P. ramorum incidence in California at fine spatial resolutions
(Kelly and Meetenmeyer 2002).
An example of the type of GIS data required for economic value transfer are GIS data
showing the location and perimeters of concentrated confirmed areas of P. ramorum
infected vegetation which were first published in January, 2002 by the sudden oak
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GENERAL TECHNICAL REPORT PSW-GTR-214
death project, Center for Assessment and Monitoring of Forest Environment
Resources (CAMFER), University of California Berkeley (figure 1). Data on land use
classes are also available on the same website. The data show 22 distinct areas of
P. ramorum infestation within Marin County, California which range in size from
46.2 to 10,126 ha, and total approximately 30,568 ha countywide. Of this total,
approximately 15,002 ha fall within local, state, and federal park land and 11,349 ha
fall within the urban residential land use class (greater than 1 housing unit per acre).
Estimates of the average annual number, area, or percentage of trees that die from
P. ramorum infections would allow estimates of non-market economic losses to be
computed. A good example is provided by Kelly and Meetenmeyer (2002) who
estimated that about six dead and dying trees per hectare could be identified in and
around China Camp State Park in Marin County, California.
Figure 1—GIS map showing concentrated confirmed areas of P. ramorum in the San
Raphael – Novato community, Marin County, California. The approximate perimeters
of infested areas are shown as grey polygons. Darkly shaded areas are urban
parcels, and the lightly shaded areas are public parks.
Conclusion
This paper argues that economic value transfer methodology can be used to provide
second-best estimates of damages to non-market economic values associated with
changes in forest health due to P. ramorum infections in California. This research
program requires data on non-market values provided by the literature in combination
with spatially reference GIS data showing the location of trees currently infected with
the pathogen, regions at risk of future dispersal of the pathogen, anticipated rates of
dispersal, and estimates of mortality rates. Although it is thought that the value
transfer methodology could be used to evaluate the non-market economic impacts of
P. ramorum throughout California, it is recommended that a preliminary evaluation
of this method be conducted by completing a spatially explicit economic value
transfer in one or a few counties of the state that are currently experiencing severe
impacts from this exotic forest pathogen.
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